Conventionally, in general car engines, fuel is injected from a fuel injection valve into an intake port, so that a homogeneous mixture of the fuel and the air is supplied to a combustion chamber. In such engines, an intake path is opened or closed by a throttle valve which is activated in response to the operation of an accelerator. The opening or closing of the intake path serves to adjust the amount of intake air (resultantly, the amount of the homogeneous mixture of the fuel and the air) supplied to the combustion chamber of the engine, thus to control the engine power.
In such a so-called homogeneous burning technique, however, a large intake negative pressure is generated due to the throttling operation of the throttle valve, thereby increasing pumping loss and reducing the efficiency. A technique called "stratified burning" which overcomes the above problem is known, where the throttling of a throttle valve is reduced and fuel is directly supplied to a combustion chamber. This enables combustible mixed air to exist in the vicinity of an ignition plug, increasing the air fuel ratio in this region to improve the ignitability.
An engine employing the above technique is provided with a fuel injection valve for stratified burning which injects fuel directing to the region in the vicinity of the ignition plug. During a low load of the engine, fuel is injected from the fuel injection valve for stratified burning so that the fuel is locally supplied in the vicinity of the ignition plug. Simultaneously with this local supply of the fuel, a throttle valve is opened, to realize the "stratified burning" described above. This enables reducing fuel consumption as well as reducing pumping loss.
Such an engine is provided with an exhaust gas recirculation (EGR) mechanism to achieve emission reduction, as in a device described in Japanese Laid-Open Publication No. 7-119513, for example. Such an EGR mechanism includes an EGR path for associating an exhaust path and an intake path of the engine with each other and an EGR valve for opening or closing the EGR path. When the engine operates in a low load range, the opening/closing of the EGR valve is controlled so that as the load increases the amount of exhaust gas to be recirculated (the EGR amount) is reduced. This control enables reducing the air excess rate and thus improving the performance of cleaning of nitrogen oxide in a catalyst device.
The above publication also discloses a technique where torque variation (output variation) of an engine is approximated to a target value by controlling the EGR amount, so as to achieve both suppression of torque variation and reduction of emission. More specifically, when the torque variation exceeds a target value, the EGR amount is decreased to suppress the torque variation to below the target value. When the torque variation becomes less than the target value, the EGR amount is increased to approximate the torque variation to the target value, and thus to reduce the amount of nitrogen oxide emitted from the engine.
During the above "stratified burning", the relationship between the EGR amount and the torque variation is as shown in FIG. 22 when the fuel injection amount is fixed. More specifically, in an area of a smaller EGR amount, the change in torque variation with increase or decrease of the EGR amount is small. In an area of a larger EGR amount, the change in torque variation with increase or decrease of the EGR amount is large. The reason why the change in torque variation with increase or decrease of the EGR amount is small in the area of a smaller EGR amount is that, since a mixed gas with a high-concentration fuel exists in the vicinity of an ignition plug in the "stratified burning", the combustion state of such a mixed gas is not easily changed with a change in the EGR amount.
In the above relationship between the EGR amount and the torque variation, assume that the EGR amount is at point A in FIG. 22. In this case, the torque variation is considerably smaller than the target value. In order to approximate the torque variation to the target value, the control is made to increase the EGR amount. This results in the torque variation shifting as shown by the solid line in FIG. 22 from point A toward the target value, i.e., point B.
In the above case, however, the torque variation increase only at a small change rate as the EGR amount is increased in the area of a smaller EGR amount as described above. It is therefore impossible to quickly approximate the torque variation to the target value.
On the contrary, in the area of a larger EGR amount, the torque variation excessively shifts as the EGR amount changes. This makes it difficult to correctly control the torque variation to be approximated to the target value. To describe more precisely, due to a response delay of the change in the EGR amount, the EGR amount does not immediately change in response to a change in the opening of the EGR valve. Accordingly, when the torque variation exceeds the target value (arrow C in FIG. 22), the torque variation exhibits a sharp increase from the target value due to the response delay at the start of decrease of the EGR amount. Such an excessive torque variation may cause a reduction of drivability.
In recent years, in order to reduce fuel consumption, the air fuel ratio of a homogeneous mixed gas to be supplied to a combustion chamber is made larger than a theoretical air fuel ratio, and swirl is generated in the mixed gas, so as to perform "lean burning".
During the "lean burning", as the EGR amount increases, the ignitability of the mixed gas decreases and simultaneously the flame speed in the mixed gas decreases. Accordingly, if the EGR amount excessively increases, the mixed gas may not be ignited or may fail to be burned out if ignited, resulting in increasing the torque variation of the engine.
During the "lean burning", also, the amount of fuel included in the mixed gas per unit volume decreases. As a result, the torque variation of the engine increases or decreases in response to a change in the fuel injection amount from a fuel injection valve. For example, the torque variation of the engine increases when the fuel injection amount falls below a required amount due to a design size tolerance for the fuel injection valve and the like. On the contrary, the torque variation of the engine decreases when the fuel injection amount exceeds the required amount due to a design size tolerance for the fuel injection valve and the like.
According to the above conventional technique, when the torque variation increases due to a decrease of the fuel injection amount below the required amount due to a design size tolerance and the like, it is attempted to suppress the torque variation by decreasing the EGR amount to be approximated to the target value. However, since the above increase of the torque variation is caused by the decrease of the fuel injection amount due to a design size tolerance for the fuel injection valve and the like, such torque variation is not suppressed even if the EGR amount is decreased. Surging generated in the engine due to the torque variation is not suppressed, either. Moreover, since the emission amount of nitrogen oxide (NO.sub.x) increases due to the decrease of the EGR amount, emission is worsened.